Hi @JimDog -
As I understand things, there are different varieties of clay in the UK and, IIRC, the worst is sometimes badged as ‘London clay’ (there are maps). And, yes, clay isn’t very permeable and retains moisture all year round (all that differs is the level of moisture, hence why with heavy clay** you can get material heave & slip - the former especially if you remove trees).
**the kind where the builder digs say a 1.5M deep trench and it fills with 500mm of ground water almost immediately!
A couple of my neighbours when building extensions have had to install concrete doughnut soakaways for their revised rainwater drainage and both had water in them when inspected by Building Control even though it hadn’t rained much. Both were passed as meeting reg’s.
I’m not sure on this but I think the road sewerage and rainwater drain systems (if you have separate runs?) are connected at some point, so a to allow overrun in extreme weather.
Re feed & gutter piping:
1- my horizontal guttering is the deeper U-shaped PVC stuff, rather than the normal half-round stuff. If you are capturing from a large roof area (say >35M’2), this is better than the ‘standard stuff’. And if you have elderly guttering and are to replace it, I’d recommend installing anti-splash boards at the bottom (sarking/eaves guard boards) - you slide these up under the felt/tiles and the shaped bottom edge sits in the gutter to prevent splashes against the facia boards. So many builders when doing extensions don’t do this, and all you see is the normal roofing under-felt flapping in the wind and not sitting properly within the line of the gutter.
2- You need to check but if your soakaway is part of a new build extension etc, then you will probably need to run underground from the building line i.e. not run surface. Underground you should/must use the normally orange waste piping, which is much more robust i.e. won’t crack. When you run in (obviously in suitably sloped trenching), you normally backfill with sand/shingle around it, so as to allow some flex for ground movement.
3- when I said about no fixed covering, I meant best not to lay a concrete slab/fixed paving over the top. And with soakaways, an inspection hatch isn’t normally viable, as you cannot see what’s going on inside the bread-baskets/doughnut anyway. IME, most people, after compaction, lay paviours. In clay, even with the best back-filling & compaction, it’s likely the ground around/on top of the soakaway will drop slightly - hence paviours are easy to lift and re-lay.
As my large soakaway cannot handle the very heavy downpours we sometime now see, my contingency is for surface overload to run in to another dug-out & shingle backfilled area - but ometimes no amount of drainage can cope in truth.
You may be able to simply dress over the top with shingle etc, but be very mindful of how deep the soakaway needs to be to get a decent drop in to it, and the ground levels overall i.e. if it is over-loaded, at what point do you want the surplus to appear and, ideally, you don’t want it running back to the property.
Re the gutter/downpipe trap/box, this is the bit underground where the gutter downpipe drops in. Older ceramic traps/boxes aren’t as effective as the new plastic sheath ones IMO and are more difficult to clean out i.e. if the ceramic ones get full of sand/mulch, they can block easily and the stuff travels to the feed piping to the soakaway.
As @Mike_S suggests, if you are concerned and want a 100% job, then I’d ask a local engineer/quality builder, who is au fait with the reg’s. Getting it wrong could mean doing again – and it’s not cheap, as mine needed a digger (with an operator who knew what he was doing!), skips to cart away a lot of clay et al, plus aggregates for compaction & back-filling.
